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+Online Summer 2010
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In the Know

In the Know links readers with University of Maine faculty, students and staff with particular expertise. On page 1, UMaine oceanographer David Townsend answers your questions about red tide. On page 2, Cooperative Extension Educator Barbara Murphy answers your questions about what’s next after the garden is planted.


David Townsend

David Townsend

David Townsend is a School of Marine Sciences professor of oceanography. His current research projects focus on phytoplankton, nutrients and red tides in the Gulf of Maine. Much of his work is based on shipboard oceanographic surveys. The most recent research cruise just returned on May 10, with three more cruises to follow (May 25-June 4; June 27-July 5; and July 29-Aug. 6). All cruises are surveying the distributions of Alerxandrium fundyense (the dinoflagellate that causes PSP) in the Georges Bank and Gulf of Maine region.

What’s most important for the public know about red tide?

There are a couple important points to keep straight:  First, there is almost never any red-colored water associated with “red tides.” They are extremely rare, for reasons I’ll explain; but the name has stuck, unfortunately.  What we are really concerned with is PSP, or Paralytic Shellfish Poisoning, which may or may not be associated with an actual bloom of the causative organism.  So, there are three terms to keep straight:  PSP event, Alexandrium fundyense bloom, and red tide.

  • PSP events occur when shellfish have ingested Alexandrium cells; the shellfish are usually unaffected, but they tend to bioaccumulate the PSP saxitoxin, which is a neurotoxin that causes paralysis in humans, especially those muscles used for breathing.  (Fish and marine mammals may also be affected if they ingest zooplankton that have fed on the Alexandrium cells).  PSP shellfish events commonly happen in summer in Maine, and it is a natural phenomenon not associated with any known human activities (such as pollution).  The causative organism, Alexandrium, is naturally occurring.  Fortunately for us, the Maine Department of Marine Resources does an outstanding job of monitoring the coast, closing areas that become toxic. I, personally, do not avoid eating shellfish in summer here in Maine — if purchased from a dealer or restaurant. DMR’s monitoring keeps Maine’s seafood safe.
  • Alexandrium fundyense is a single-celled photosynthetic alga belonging to a group of phytoplankton known as dinoflagellates. They are microscopic, only about 35 micrometers in diameter, and are barely visible to the naked eye — if you can have very good vision.  They are planktonic in that they have only very limited swimming abilities; by beating their pair of flagellae, they can move vertically a few meters per day, but they generally drift with the ocean currents.  A. fundyense occurs naturally in the Gulf of Maine region and usually does not grow to very high cell densities, as compared with other species of phytoplankton. But sometimes it does.  Cell densities of only 100 to 200 cells per liter in the ocean are needed for shellfish to become toxic to humans — especially mussels, which filter more water than soft shell clams, and so they concentrate the cells and toxin more readily. However, sometimes those cell densities get much higher — several tens of thousands of cells per liter — in which case the shellfish can become very toxic.  Red water is only visible when the cell densities reach close to a million cells per liter, which, I believe, only occurs when the cells concentrate right at the surface on a calm day, just before they exchange gametes to produce a resting stage, a cyst, that drops to the deep and bottom waters, awaiting next year’s growth cycle.
  • As just explained, a “bloom” of A. fundyense — that is, when they reach very high cell densities — is not necessary for there to be a PSP problem in Maine’s shellfish.  But, almost every year they do bloom well offshore.  This was a major discovery we made some 12 years ago as part of our ongoing research.  Offshore of the coast in eastern Maine is the Eastern Maine Coastal Current (EMCC) made up of tidally well-mixed waters from the eastern Maine coast and Grand Manan Island area, as well as newly upwelled deep Gulf of Maine waters, that flow to the southwest and across much of the Gulf of Maine.  Those cold, nutrient waters are where A. fundyense cells seem to like to grow in summer, sometimes reaching high cell densities (we’ve seen densities as high as 100,000 cells per liter out there).  Normally, those nutrient-rich waters of the EMCC turn to the south somewhat and bring those cells offshore from the coastal shellfish beds.  But, in some years, when there are northeast winds, the surface waters will be “blown” inshore (this is because surface ocean currents flow to the right of the wind direction, which is related to the Earth’s rotation).  When we get a northeast wind, those cells sitting “harmlessly” offshore will flow to the northwest and toward shore.  And in some years, such as 2005, that coastal current system will carry cells along the entire length of the coast, to New Hampshire and Massachusetts and even beyond, all the while carrying an actively growing population of A. fundyense, and causing major PSP events.

What causes red tide?

The necessary ingredients are:

  • High light levels, such as in summer months.
  • High levels of nutrients, especially nitrogen, which occurs naturally in the Eastern Maine Coastal Current (as well as in the Bay of Fundy and out on Georges Bank).
  • A source population, which are either the resting cysts from the previous year’s growth season or the few cells that have somehow survived the winter in the plankton.

Is there a way to stop red tide from occurring?

Probably not – not legally, anyway. My colleague at Woods Hole Oceanographic, Don Anderson, has conducted a few controlled experiments based on his idea of spreading clay particles on bloom areas, which causes the cells to sort of congeal and sink.  But the blooms can be very large, covering tens to hundreds of square miles.  These phenomena are natural and have occurred for millennia, and so I believe we need to understand them better, and manage our seafood harvests accordingly in order to protect human health.

What are the human health and safety issues related to red tide?

The greatest danger, in my view, is the general assumption that offshore waters of the Gulf Maine are pristine – with no pollution problems to speak of – and so it should be OK to eat shellfish collected well offshore (on Georges Bank, for example).  The Gulf is clean, but A. fundyense grows very well here in our clean waters nonetheless (and it grows very well out on Georges Bank, for reasons we are still exploring; we will be out there again from June 30 to July 8 on the Research Vessel Oceanus studying the phenomenon).  Remember, their rich nutrient requirement is met naturally, as deep waters are mixed to the surface.  In fact, it was off Jonesport several years ago that a floating bait barrel was found several miles offshore, which was covered with mussels; they were assumed to be clean, people ate them, and became quite sick, with several having to be admitted to the hospital.

In the Gulf of Maine, where are the highest incidences of blooms?

In the Bay of Fundy, Georges Bank and coastal waters that can be traced back to the Eastern Maine Coastal Current.  In other words, because of the variability in that current, we can have problems almost anywhere.

In addition, we have some evidence that there may be a “resident” population of A. fundyense in the upper reaches if Casco Bay, where their cysts and cells do not get flushed out each year; in early spring, after diatom bloom there, we often see a spike in PSP.  But the areal extent is quite limited.  Also, a similar phenomenon of a resident population seems to occur each year in Cobscook Bay, in eastern-most Maine, which we have not investigated.

This past winter we “predicted” a possible major PSP event in the western Gulf (we are urged to issue an outlook each year by officials at our funding agency, NOAA), which thus far has not happened.  We based that prediction on the annual fall survey of the numbers of resting cysts on the bottom, a survey conducted by colleagues at Woods Hole Oceanographic Institute, which revealed very high cyst abundances, primarily in the western Gulf of Maine.  With “normal” oceanographic conditions, this could have been a bad year.  But this year, for reasons we are still researching, the Eastern Maine Coastal Current (EMCC) — the source of nutrients — was “diverted” back into the eastern Gulf of Maine.  Because it did not flow to the western Gulf, we think that cysts that “hatched” and arrived at the surface did not have that high-nutrient water waiting for them, and so there were almost no A. fundyense cells there so far this year, and therefore, no real problems with PSP.  But, we did see, as expected, increased cell densities in the eastern Gulf, in association with the EMCC.

I know red tide can close clam flats. Is there an average time for such closures? In other words, how long do blooms usually last?

Once a shellfish bed becomes toxic, as determined by the Maine DMR, it continues to be monitored until there are several “clean tests.”  This can last several weeks. Even though the A. fundyense cells may no longer be present, the toxin takes a while for the shellfish to naturally depurate.

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